The invention relates generally to semiconductor devices and integrated circuit fabrication and, in particular, to fabrication methods and device structures for a bipolar junction transistor.
Bipolar junction transistors may be found, among other end uses, in high-frequency and high-power applications. In particular, bipolar junction transistors may find specific end uses in amplifiers for wireless communications systems and mobile devices, switches, and oscillators. Bipolar junction transistors may also be used in high-speed logic circuits. Bipolar junction transistors are three-terminal electronic devices that include an emitter, an intrinsic base, and a collector defined by regions of different semiconductor materials. In the device structure, the intrinsic base is situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled by the base-emitter voltage.
Bipolar junction transistors are composed of one or more emitter fingers and they may have a non-uniform temperature profile along their finger length with a cooler temperature profile at the ends of the fingers. This non-uniform temperature may change transistor parameters, such as current gain (beta), resulting in non-uniform beta along the length of the finger. Thus, it is desirable to make the temperature profile along and among the emitter fingers more uniform.
Consequently, improved fabrication methods and device structures are needed for a bipolar junction transistor.